A spectrophotometer using a photodiode array is known from "A High-Speed Spectrophotometric LC Detector", Hewlett-Packard Journal, April 1984. This known spectrometer is used in a liquid chromatograph for analyzing the substances eluting from the chromatographic column. In this spectromter, a broad spectrum of ultraviolet and visible radiation is transmitted through a sample cell through which the sample eluting from the column passes. After transmission through the sample cell, the spectrum of the radiation as modified by the sample substances is derived by means of a diffraction grating which directs light rays of different wavelengths into different directions. A linear array of photodiodes is arranged to receive the light diffracted by the grating. Each diode thus receives light corresponding to a different wavelength range.
The photodiode array used in the mentioned spectrophotometer is of the type "selfscanning photodiode array". Such a selfscanning photodiode array is built on semiconductor material and comprises a plurality of photocells each photocell consisting of a photosensitive element (photodiode) and a capacitor which represents the junction capacitance of the photodiodes or which is separately added on the semiconductor chip. The photocells are connected via electronic switches to a common output line (video line) which in turn is connected to an external charge amplifier. The switches of the individual photocells are controlled by a shift register such that the photocells are read sequentially according to the clock signals of the shift register. In operation, the capacitors of the photocells which have initially been charged to a fixed value are discharged by the current generated by the photodiodes when light impinges on them. These capacitors are periodically recharged to the initial value whereby the amount of charge transferred is representative of the amount of light which has impinged on the photodiode during the integration period. The recharging of the capacitors is performed sequentially starting from the photocell at one end of the array to the photocell at the other end of the array. The transfer of charge to a photocell causes a voltage change at the output of the charge amplifier and thus produces a signal indicative of the light intensity on the photodiode. This signal is then converted to a digital signal by an analog-to-digital (A/D) converter common to all photodiodes of the array. The thus obtained digital signals can then be further processed by data processing means to generate a spectrum corresponding to the sample to be analyzed.
A second type of photodiode arrays are so-called "random access photodiode arrays". Their structure is similar to the selfscanning photodiode arrays in that they comprise a plurality of photocells connected to a common video line and to a subsequent common charge amplifier and an A/D converter. The difference to the selfscanning photodiode arrays consists in the way how the switches of the photocells are controlled : The shift register is replaced by an address decoder which gives the possibility to randomly select the switches to be activated so that any photocell can be chosen in any order.
A third type of photodiode arrays are so-called "multi element photodiode arrays". They typically comprise a number of less than about 50 photodiodes linearly arranged on a silicon chip without additional circuitry for operating the photodiode array on the chip. The one side of each photodiode is directly brought out from the chip while the other sides are commonly connected to the substrate on the chip . These photodiode arrays can be operated in two modes: In the first mode, each photodiode is connected to an operational amplifier which serves for signal conditioning. The output signals of these amplifiers are time multiplexed and supplied to a common A/D converter. Due to the amplifiers associated with each photocell, this configuration is very complex. In a less complex, alternative configuration, the multiplexer is arranged between the array of photodiodes and a common operational amplifier. In this configuration, however, the fidelity of the signals is impaired.
The multi element photodiode arrays thus operate in a sampling mode whereas the selfscanning and random access photodiode arrays operate in an integrating mode, i.e., the photocurrent of a photocell is accumulated during the time when the signal from a different photocell is processed.
The photodiode arrays operated in an integrating mode, i.e., selfscanning and random access photodiode arrays, lead to a problem called spectra distortion which will be explained in the following:
This problem of spectra distortion occurs in spectrophotometers wherein the sample to be analyzed is changing as a function of time. A typical example is a spectrophotometer used to detect the sample substances eluting from the column of a liquid chromatograph. In that case, different sample components are detected by the spectrophotometer at different times; furthermore, even with a pure sample substance, the corresponding sample concentration is a function of time in accordance with the chromatographic peak having a rising edge, a top, and a falling edge. Since the signals from the individual photocells in an integrating phototdiode array are processed sequentially, the photodiodes on one side of the array, corresponding to the shorter wavelengths of the spectrum, monitor the short wavelength radiation earlier than the photodiodes on the other side monitor the long wavelength radiation. When monitoring a chromatographic peak corresponding to a certain substance separated in the column, the resulting wavelength spectrum obtained at the top of such a peak (apex spectrum) is different to the spectrum obtained at the rising edge of the peak (upslope spectrum) which again is different to the spectrum obtained at the falling edge of the peak. In the upslope spectrum, the higher wavelengths are over-accentuated because the sample concentration rises during the spectrum accumulation which proceeds from lower to higher wavelengths. In the downslope spectrum, the lower wavelengths are over-accentuated because the sample concentration decreases so that upon processing of the higher wavelengths a smaller sample concentration prevails than at the processing of the lower wavelengths.
Since sample substances are identified by comparison of the measured spectra with spectra stored in an electronic library, the above described spectra distortion limits the capabilities to identify unknown sample substances.
In liquid chromatography, it may occur that two different sample constituents are not completely separated in the chromatographic column so that the corresponding peaks in the chromatogram are not resolved but form a single peak. A method for determining if a peak in a chromatogram represents a single or more constituents is the "peak purity check". This peak purity check consists in the comparison of the upslope wavelength spectrum of a peak with the downslope spectrum at the corresponding sample concentration. If both spectra coincide closely, there is a confirmation that the chromatographic peak represents a single constituent. However, if the spectral scans do not coincide, there is a strong indication that another constituent is contributing to the peak. Due to the above mentioned spectra distortion, the peak purity check can only be applied if the change in sample concentration during the integration interval is small or if the sample concentration is kept constant for a while by stopping the flow of the sample substances. The stopping of the flow through the separation column and the sample cell, however, is often not desirable because this introduces disturbances which impair the quantitative accuracy of the chromatographic measurement.
A prior art approach for coping with the described problem of spectra distortion is described in EP-A-0 192 200. According to this prior art, the photodiode array is of the type multi element photodiode array having 35 diodes wherein each photodiode is connected to an operational amplifier for signal conditioning and a sample and hold circuit for sampling and holding the output signals of the operational amplifiers. The output signals of the sample and hold circuits are multiplexed by a multiplex switch and then sequentially converted by a single A/D converter into digital signals. The circuitry of this known device is very complex because it requires an operational amplifier and a sample and hold circuit for each of the photodiodes as well as a multiplexer with a number of channels corresponding to the number of photodiodes and a high performance A/D converter. Furthermore, all electronic components are built discretely on a printed circuit board thus requiring a large board space and a great amount of supply power.
An additional problem with known photodiode arrays originates from the A/D conversion of the signals corresponding to the light incident on the photodiodes. In all of the above mentioned types of photodiode arrays, a single A/D converter is used which successively converts the signals from the individual photodiodes. Since the number of photodiodes used in spectrophotometic applications is very large (up to 1024 photodiodes), the conversion rate of the A/D converter has to be very high, e.g., above 100 kHz, in order to achieve a time resolution of 100 data points per second for each photodiode. Furthermore, in order to ensure high measuring accuracy in spectrophotometric applications, the resolution has to be high (preferably larger than 16 bit) and a good linearity must be ensured. In order to fully meet these requirements, a complex and expensive A/D converter would be required. As a compromise between speed and resolution on one side and cost and complexity on the other side, most prior art photodiode arrays use A/D converters of the type "successive approximation". Such A/D converters, however, suffer from differential non-linearities, known as "missing codes". Such errors in the A/D conversion limit the detection limit in spectrophotometry for chemical analysis where small differences in intensity have to be measured.